Structures Repair Goes Ultrasonic

To increase the safety and useful life of your steel bridges and steel
traffic structures, consider going ultrasonic. Ultrasonic impact treatment
(UIT) helps prevent fatigue cracking in welds and welded members of
new bridges and structures by enhancing the geometry of the weld, introducing
favorable compressive stresses, and relieving tensile stresses internal
to the weld. Eliminating the fatigue cracking not only increases service
life but reduces the overall lifetime cost of maintenance. UIT can also
improve the performance of existing structures. Many in-service steel
bridges and traffic structures were built before fatigue was understood
and therefore have fatigue-prone details, such as cover plates and socket
joints. Structure owners must continually monitor these details and
often must retrofit them, which is costly and not always effective.
Applying UIT minimizes the need for retrofits and extends the life of
the structure.

UIT is performed using a hand tool that weighs approximately 8 pounds.
This tool is attached to an electronic control box and water cooling
system. Treatments can be applied under normal operating conditions,
resulting in minimal or no traffic disruptions. UIT also produces little
noise, as opposed to other weld treatments, such as hammer peening.
The procedure can be carried out in approximately one-tenth the time
needed for conventional retrofit procedures.

UIT is applied to a traffic signal mast arm.

The use of UIT can allow steel structure designers to employ fatigue-prone
details that they would otherwise have to avoid, such as socket joints.
UIT can be applied to the socket joint welds, for example, removing
fatigue as a concern for that detail. As fatigue-resistant designs are
more difficult to construct and more expensive, use of UIT can allow
engineers to use more cost-effective designs that are still fatigue
resistant.

UIT was invented in 1972 by Dr. Efim Statnikov and used by the Soviet
Union in its Naval and Aerospace programs. The technology was first
demonstrated in the United States in 1995. In 1996, William Wright of
the Federal Highway Administration (FHWA) conducted tests using UIT
on full size girder members at FHWA's Turner-Fairbank Highway Research
Center in McLean, Virginia. This led to an FHWA-funded research program
at Lehigh University, which has studied the effects of UIT on more than
23 full size structural beams. The work, which is also sponsored by
the Pennsylvania Department of Transportation, is being performed by
Dr. John Fisher. The results of the studies "conclusively demonstrate
that the use of UIT technology significantly extends the fatigue life
of various road and highway related structures," says Krishna Verma
of FHWA. In 2002, FHWA extended the program's funding to cover further
tests on the use of UIT on bridge structures employing advanced, high
strength steels. "By increasing the fatigue categorization of the
welds, designers can take greater advantage of the improved mechanical
properties of the steel," says Verma. The Texas Department of Transportation
(TxDOT) has also sponsored research at the University of Texas-Austin
that examined the fatigue life improvement by UIT of socket joints,
which are found on thousands of fatigue-prone traffic structures nationwide.

UIT has been tested on bridges in Georgia and Virginia. "These
applications demonstrated that UIT could easily be applied not only
in a manufacturing environment but also in the field as a standard maintenance
and repair technique," says Verma. In Georgia, the technology was
used on a highway bridge at Allatoona Lake that had been in service
for 20 years. Numerous fatigue cracks had been found at the main girder
beams. Repairs were performed by hole-drilling or welding, following
by application of UIT. Some undamaged parts of the bridge were also
treated with UIT to reduce the chances of future cracking. In follow-up monitoring, the
Georgia Department of Transportation found that UIT had been effective in improving
the fatigue performance of the weld repairs.

FHWA and individual State highway agencies have also sponsored demonstrations
of the technology across the country. TxDOT is currently sponsoring
a research program at the University of Texas that is testing fatigue
treatments for traffic signal mast arms. The preliminary results, as
reported at the 2003 Transportation Research Board Annual Meeting, show
that when treated by UIT under dead-load conditions, mast arm socket
joints can be improved by several fatigue categories. TxDOT plans to
treat fatigue prone structures with UIT and is also looking into improving
the fatigue life of new traffic structures.